A diffusion transfer photographic process utilizing a silver salt such as a silver halide is hitherto known, which comprises superimposing a photosensitive element containing an exposed silver halide photographic emulsion on an image-receiving element containing a silver precipitating agent, and coating the opposing surfaces of the two elements with an alkaline processing solution containing a solvent for the silver halide in the presence of a developing agent, thereby forming a positive silver image directly on the image-receiving element. In this method, the unexposed silver halide emulsion in the photosensitive element is dissolved by the solvent for the silver halide and moves to the alkaline processing solution in a form of silver ion complex. Consequently, the silver ion complex is transferred to the image-receiving element and by the action of the silver precipitating agent, is precipitated in the image-receiving element as a silver image. A positive image is thus directly formed.
The image-receiving element used in this method is usually produced by providing an image-receiving layer containing a silver precipitating agent selected from metal sulfides such as nickel sulfide, silver sulfide and palladium sulfide or noble metal colloids such as gold, silver and palladium colloids in an alkali-permeable polymer binder selected from gelatin, carboxy methyl cellulose, hydroxyethyl cellulose, regenerated cellulose, polyvinyl alcohol, sodium alginate, starch, gum arabic and colloidal silica on a paper support such as baryta paper, polyethylene laminated paper, lacquered paper or a synthetic paper-like sheet or a film support such as an acetylcellulose film, a polyethylene terephthalate film or a polystyrene film.
Many attempts have been made to improve such image-forming elements, including inventions directed to the use of regenerated cellulose as a binder for the image-receiving layer.
U.S. Pat. No. 3,179,517 discloses a method of preparing an image-receiving element which comprises hydrolyzing an acetyl cellulose film in the presence of alkali to form a layer of regenerated cellulose, dipping the regenerated cellulose layer in a gold salt solution and a solution of a reducing agent, and thus reacting them in the regenerated cellulose layer to form a silver precipitating agent composed of gold colloid. Japanese Patent Publication No. 32754/69 (corresponding to U.S. Pat. No. 3,671,241) discloses an image-receiving element prepared by including a silver precipitating agent into an alkali-impermeable polymeric substance by vacuum deposition, dissolving it in a solvent capable of dissolving the polymeric substance, coating the solution on a support, drying the coated support and hydrolyzing the surface layer of the resulting polymeric layer to make it alkali-permeable.
Japanese Patent Publication No. 43944/71 (corresponding to U.S. Pat. No. 3,772,025) describes a method of preparing an image-receiving element which comprises forming a silver precipitating agent in an acetyl cellulose solution, coating the solution on a support, and thereafter hydrolyzing the acetyl cellulose to form a regenerated cellulose.
Japanese Patent Publication No. 49411/76 describes an image-receiving element prepared by hydrolyzing a cellulose ester layer, and including a silver precipitating agent into the hydrolyzed layer simultaneously with or after the hydrolysis.
U.S. Pat. No. 4,163,816 describes an image-receiving element prepared by hydrolyzing acetyl cellulose with an acid in solution to convert it into acetyl cellulose of a low degree of acetylation, and coating the resulting solution on a support.
The image-receiving elements obtained by these prior techniques, however, have the disadvantage that silver images formed on these elements are susceptible to discoloration or fading.
Attempts to improve this disadvantage are disclosed in U.S. Pat. No. 3,533,789, Japanese Patent Publication No. 5392/71 (corresponding to U.S. Pat. No. 3,533,789), and British Patent No. 1,164,642 which relate to a method involving coating a water-soluble polymer solution containing an alkali neutralizing component on the surface of image-receiving element forming a silver image. According to this method, a considerable period of time is required until the surface coated with the aqueous polymer solution completely dries. During this time, the surface is sticky and one element cannot be put upon another. Moreover, fingerprints or dust frequently adhere to the surface, and the additional step of coating of such a solution on the silver image is required.
Japanese Patent Publication No. 44418/81 (corresponding to U.S. Pat. No. 3,607,269) discloses an image-receiving element comprising a support and formed thereon (I) a cellulose ester, polyvinyl ester or polyvinyl acetyl layer which is hydrolyzable and becomes alkali-permeable upon hydrolysis and contains a compound capable of diffusing and changing the nature of a silver image and (II) a regenerated cellulose layer containing a silver precipitating agent, in this sequence. It describes organic mercapto compounds as the compound capable of diffusing and changing the nature of the silver image. With this image-receiving element, the mercapto compound in layer I gradually diffuses into layer II after the diffusion transfer treatment, and thus protects the silver image formed in layer II and prevents its discoloration or fading.
In order for the above image-receiving element to completely prevent discoloration or fading, the mercapto compound must have a sufficient ability to prevent discoloration or fading; stay in layer I during the storage of the undeveloped image-receiving layer and the diffusion transfer treatment; permeate from layer I to layer II after the formation of a silver image by the diffusion transfer treatment and thus protect the image formed in the layer II. If the diffusion of the mercapto compound from layer I to layer II occurs before the completion of the diffusion transfer treatment, development is inhibited and the optical density of the transferred silver image on the image-receiving element is decreased. Furthermore, if the diffusion of the mercapto compound is too slow, discoloration or fading of the silver image occurs before it is protected by the mercapto compound.
The mercapto compounds described in Japanese Patent Publication No. 44418/81 have the defect that their ability to prevent discoloration or fading is insufficient and consequently the image undergoes discoloration or fading, and the further disadvantage that during storage of undeveloped image-receiving elements, the mercapto compounds diffuse from the layer I to the layer II to inhibit development and reduce the optical density of the transferred silver image.
Japanese Patent Application (OPI) No. 120634/74 (corresponding to U.S. Pat. No. 3,963,495) (the term "OPI" as used herein refers to a "published, unexamined Japanese patent application"), describes an image-receiving element prepared by using a homopolymer, copolymer or graft copolymer of a monoacrylate or monomethacrylate of a polyhydric alcohol as a polymer layer containing a compound capable of changing the nature of a silver image. The compound described, as is the case with the compounds described in Japanese Patent Publication No. 44418/81 (corresponding to U.S. Pat. No. 3,607,269), has an insufficient ability to prevent the discoloration or fading of images, or causes a decrease in the optical density of the transferred silver image during storage of undeveloped image-receiving elements.
British Patent No. 1,276,961 discloses the use of 2-mercapto-1,3,4-triazole derivatives for obtaining stable silver images by the diffusion transfer process. U.S. Pat. No. 3,655,380 discloses that 5-seleno-1,2,3,4-tetrazole derivatives not only can modify the color of a silver image obtained by a diffusion transfer process to a nutral gray color, but also provide stable silver images. These compounds, however, have the defect that the stabilization of silver image produced by the diffusion transfer process is insufficient, and the images undergo discoloration or fading. It is desired to develop compounds which have a great stabilizing effect on silver images, and which do not adversely affect undeveloped image-receiving elements during storage, and image-receiving elements containing such compounds.